Lamp modulated recording method and system



Oct. 10, 1944. H. REISKIND 2,350,012

LAMP MODULATED RECORDING METHOD AND SYSTEM Filed Dec. 50, 1942 2 Sheets-Sheet l llllllll 'III III ELLEL .ZZEE/sK/ND,

INTOR.

ATTORNEY.

Oct. 10, 1944. I RElSKlND 7 2,360,012

LAMP MODULATED RECORDING METHOD ND SYSTEM Filed Dec 50, 1942 2 Sheets-Sheet 2 n Alllllllllll 'IITII'" g? L INVENTOR.

BY Wifiw ATTORNEY.

-- termined level.

wherein two feedback paths Patented Oct. 10, 1944' RECORDING METHOD LAMP MODULATED AND SYSTEM.

Hillel I. .Reiskind, Indianapolis. Ind..- assignor to Radio Corporation Delaware 1 America. a corp ration of Application December 30, 1942, Serial No. 479,663

10 Claims. (01. lie-100.3) I

invention relates to sound recording and reproducing systems and particularly to a stabilizing system for light sources employed in sound recording systems.

It is well known that in photographic sound recording system, it is necessary to employ a light source which may or may not be directly modulated by sound waves to be recorded, while in a sound reproducing system a light source of constant intensity is used toscan the recorded sound track. In variable density recording sys-.

terns, it is common practice to employ a light source whose light output or quanta, as impressed upon the film: is varied by varying the voltage thereon in accordance with the sound waves being recorded. The present invention is particularly applicable to the stabilization of such light sources since any variations in their light output, not produced by the sound waves, will introduce distortion 'or vary the average prede-v J Unwanted light variations may be produced by fluctuations in the lamp energizing source, by deterioration of the light producing elements, blackening of the lamp envelope and other reasons.

It has been found that the maintenance of a constant voltage at the lamp terminals does not compensate for variation in lamp temperature, deterioration of electrodes, and certain. other causes of unwanted light variations mentioned above. The present invention, therefore, obtains stabilization by detecting a portion of the light output from the lamp and regulating the energy to the lamp in accordance with the variations in the detected light. The general principle of such an optical feedback stabilizing system is known in the art, such a system being disclosed and an optical feedback system for stabilizing direct current amplifier claimed in U. 3. Patent No. 2,242,638, of May 20,

1941. In this patented system which provides direct current feedback, the impedance of an electronic device connected in series with the lamp and the primary accordance with the light variations. The amount of feedback insuch. a system is limited by the size of the photocell load resistor and the, load resistor of the first vacuum tube and when these resistors are increased to increase the.ieedpower source is varied in through a partially back, the effect of stray capacity is also increased,

and it is thus difficult to obtain sufllcient' amplification in the feedback circuit with a linear frequency characteristic.

The present invention is directed to a system are provided in parpath of moderate gain allel, one a direct current path of highand the other an alternating current gain, the outputs of the two paths providing an accurate and sensitive control of the'average.

tortion and maximum sensitivity.

Another object of the invention is to prciivide a ght source for a sound recording system wherein the energy supplied ect of the invention, therelight source stabilizing sysmodulating input is transmitted by the feedback circuits. Although to be characteristic of out with particularity in the claims appended herewith, the manner of'its organization and the mode of its operation referring too the following description, read in conjunction with the accompanying drawings. in which: Fig. l is ablock diagram of the system embodying the inventionI Fig. 2 is aschematic diagram of the circuits used in the system' ofFig. 1, and,

Fig.- 3 is a partial schematic circuit showing modifications of the modulating portion of the system.

Referring now to Fig. 1, may be a mercury vapor gas fed from a modulator 6 which nected over conductors 8 withthe output of a current amplifier ill. The inputs of the ampli flers 9 and III are in parallel with the photoelectric cell l2 and terminals it, these terminals being connectable to a microphone or other source of modulating signal. The photoelectric cell l2 receives light from the lamp 5 transmitted reflecting mirror I, the reflected portion of the light being impressed upon a film It when the lamp 5 is used in a sound recording system.

Referring now to Fig. 2, the light source 5 and photoelectric cell i: are shown in an arrangement whereby a portion of the light from lamp- 5 is impressed on the cell I: as in Fig. 1. The output of-the cell I2 is impressed on the first grid of' the inventionis to provide anv will be better understood by t a light source, Ii which discharge lamp is. in turn iscon 9 and an alternatingcurrent v over conductors l9, coupling'condenser 20 and resistor 2|. The modulating signal is impressed on the same grids in series with resistor it when a signal source is connected to terminals 25. A noise reduction signal, if desired, may be impressed across resistor (I in series with a polarizing source 48 when connected to terminals 49.

The output of thefirst tube l8 of the altemating current amplifier is impressed upon a second tube 28 over condenser 29, while the output of the second tube 28 is impressed upon a third powertube 8| over condenser 82. The output of the tube 8! is connected over condenser 34 to the output of direct current amplifier tube l at point 35, point 35 being connected to the control grid of a modulator tube 88. The output of the tube is connected to the lamp over conductors 88 and resistor It. The proper anode potentials for the tubes'i5 and 36 are obtained from the plus B terminal 88 over resistors 44 and 45 while the anode potentials for the A. C. amplifier tubes- I8, 28 and 3! are supplied over their respective plate resistances Ii, 42 and 48, as indicated.

In operation the output of photoelectric cell I2 is impressed on tubes i5 and I8, the output of tube 18 after amplification in tubes 28 and 3| being impressed in parallel with the output of tube ii on power tube 88. The input coupling resistor It is low enough so that the high he quency attenuation in the coupling circuit is kept at a reasonably low value. Tubes l5 and 3| are high impedance pentodes and thus function-as constant current generators permitting the outputs thereof to be connected in parallel. Since two high impedance voltage sources are connected in parallel to a load of relatively low impedance, the voltage across resistor 38 will be equal to the sum of the output voltages of the tubes i5 and 8i, Thus, the eflect of the circuit of Fig. 2 is thesame as that of Fig. 1. In this manner the alternating current feedback path including tubes i8, 28 and 8| will respond to rapid variations in light intensity and thus compensate therefor, while the direct current path including tube I I will compensate for the slower light variations, the two outputs being additive to maintain the light output of lamp 5 at its predetermined light output average constancy.

combining the output currents in said transmisslOn paths for impression on said light source.

2. The method of controlling the light intensity of a light source in accordance with claim 1 into electrical currents, separately amplifying in which one of said paths transmits the components of said translated currents corresponding to the slow variations in light from said source and the other path transmits the components of said translated currents corresponding to the rapid variations in the light from said source.

3. The method of controlling the light intensity of a modulated light source comprising translating a portion of the light from said source pliiying said translated currents, combining said separately amplified currents, impressing said combined currents on said light source, and combining a modulating signal with said separately amplified currents.

5. A stabilizing system for a light source comprising means for translating a portion of the light from said source into corresponding electrical currents, a plurality of transmission paths interconnecting the output of said translating device with the input of said light source, and means for combining the outputs of said transmission paths for impression on said light source.

6. A stabilizing system for a light source comprising means for translating a portion of the light from said source into corresponding electrical currents, a plurality of transmission paths interconnecting the output of said translating devicewith the input of said light source, and means for combining the outputs of said transmission paths for impression on said light source,

one of said paths includinga direct current amalternating current amplifier.

' 7. A stabilizing system for a light source complifier and the other of'said paths including an prising means for translating a portion of the .To eliminate the effect of the time constant of the coupling elements 28- in the input circuit of the first tube 18 of the A. C. amplifier, this stagemay be connected as a' D. 0. stage, as shown in Fig. 3. The same numerals .in Fig. 3 identify corresponding elementsinl'ig. 2. i

The above feedback stabilizing and light light from said source into corresponding electrical currents, a" plurality of transmission paths interconnecting the output of. said translating device with the input of said light source, and

means for combining the outputs of said transmission paths for impression on said light source,

modulating system provides a sensitive andrapid control of the output of a light source by providing a pair oifeedback paths, one an alternating current path to control the lamp during rapid light fluctuations and having sumcient gain to produce the required amount of feedback to give a flat frequency characteristic and low distortion, and the other a direct current path of moderate gain to control thelamp during slower light variations and providing stabilization of the av- I erage. illumination. In this manner, high irequency attenuation and distortion are eliminated tin the above describedtype of sound recording systems What is claimed is:

1. The method of controlling the light intensit of a light source comprising translating a portion of the light from said source into electrical currents, transmitting said electrical currents over two independent on paths, and

and,means for impressing a signal on one of said transmission paths.

8. A stabilizing system fora modulated light source employed in recording sound comprising means for utilizing a portion of the light from said source for recording sound and another portion of said light for generating electrical currents, a plurality-of transmission circuits for said generated electrical currents, means for combining the output of said transmission circuits for impression .on said light source, one of said transmission circuits amplifying-the components of said generated currents corresponding to the slow variations in light from said source, and the other of said transmission circuits amplifying the components of said generated currents corresponding to the more rapid variationsin light from said source, and means for impressing a signal to be recorded on said transmissioncircuits.

9. A stabilizing system in accordance with claim 8 in which the output impedance of said transmission circuits are high relative to the 10nd impedance, said outputs being connected in parallei,

10. In combination, a light source adapted to be modulated about a point of predetermined intensity, means for modulating said light source,

means for generating currents from said light source, means automatically energy controlling 

